The present invention relates to an optical device comprising an optical source adapted to emit a light beam in a plurality of emitting wavelengths, at least one laser, each of said at least one laser including a holographic medium, means for injecting said light beam derived from said optical source into said at least one laser, said holographic medium being adapted to generate a hologram, so as to oscillate said at least one laser in at least one oscillation mode, said at least one oscillation mode being determined by at least one excitation wavelength among said plurality of emitting wavelengths.
Such type of device is known for example, from the publication by N. Dubreuil, G. Pauliat and G. Roosen “Tuneable Self-Adaptive Laser Diode with Wavelength All-Optical Addressing” (ECOC 2004 Proceedings—Vol. 3—Paper We4.P.065—p. 600-601), in which the optical source used is a tuneable so-called “master laser”, a source of light having an important coherence length. Such master laser is used for addressing a cavity of a slave laser through the recording in a holographic material of the cavity. More precisely, when the beam derived from the master laser is injected into the cavity of the slave laser, the spectrum emitted by the latter is modified. The new structure of modes, which oscillates in the cavity of the slave laser, records a hologram in the dynamic holographic medium. Such hologram reduces the losses of the mode or the modes, which oscillates or oscillate in the cavity of the slave laser and enables to strengthen the operation of the slave laser in a particular mode. Upon cutting off the beam derived from the master source, the hologram persists and enables the slave laser to keep the wavelength imparted by the master source during the injection. Thus, the wavelength of the slave laser is determined by the optical injection of a beam derived from the master source and stored by the dynamic holographic medium inserted into the cavity of the slave laser. Such wavelength is then kept after the cut-off of the beam derived from the master slave. Thus, it is possible to address a second slave laser, possibly on a different wavelength, the first slave laser still oscillating according to the fixed wavelength.
However, this advantageous addressing method of a slave laser by a master laser has a certain number of disadvantages. First, when a master laser is used, it is necessary to adjust the wavelength of the laser in the neighbourhood of a mode of the slave laser cavity with the utmost accuracy, so that the injection condition is complied with. If the wavelength of the master laser is too far from that of a mode of the slave cavity, there won't be any destabilisation of the operation of the slave laser towards an operation in a wavelength neighbouring that of the master laser. It will be understood that such required adjustment of the wavelength of the tuneable laser is a disadvantage of the mounting such as described in the hereabove-mentioned publication.
A second disadvantage of the device, according to the prior art as mentioned hereabove, relates to the polarisation constraints between the master laser and the slave laser or lasers. As a matter of fact, in the case where the polarisation condition of the master laser is perpendicular to the polarisation condition of the slave laser, the operation of the slave laser is not disturbed by the beam derived from the master laser and no addressing can be performed. This is the reason why the mounting, according to the prior art mentioned hereabove, requires a polarisation tester in order to at least avoid the perpendicularity of the polarisation of the two lasers. It should be understood that the necessity of having such a tester is a disadvantage of the mounting according to the prior art mentioned above. In addition, in the situation where the physical relationship between the master laser and the slave laser is provided using an optical fibre, the fibre introduces a birefringence, which results in an action on the polarisation condition of the beams. It is thus necessary to compensate for such variations in birefringence by systematically adjusting the polarisation tester. It will thus be advantageous to eliminate the polarisation constraints between the master source and the slave cavity. A third disadvantage of the device, according to the prior art mentioned hereabove, is the cost of the single mode laser used, as well as the relative complexity of the mounting.
One of the aims of the present invention is thus to remedy at least one of the hereabove-mentioned disadvantages. For this purpose, the present invention provides an optical device comprising an optical source adapted to emit a light beam in a plurality of emitting wavelengths and at least one laser, each of said at least one laser comprising a holographic medium, means for injecting said light beam derived from said optical source into said at least one laser, said holographic medium being adapted to generate a laser, so as to oscillate said at least one laser in at least one oscillation mode, said at least one oscillation mode being determined by at least one excitation wavelength among the plurality of emitting wavelengths, characterised in that said optical force includes a light source simultaneously emitting in said plurality of emitting wavelengths.
In the following, said optical source comprising a light source simultaneously emitting in said plurality of emitting wavelengths will be designated as a “wide-band source”. The plurality of emitting wavelengths can be simultaneously generated in a discreet or continuous way. An example of such a wide-band source is a source producing light by means of an amplified spontaneous emission process (ESA).
For example, let us assume an addressing in the wavelengths of a slave laser according to a comb of several wavelengths separated by 0.4 nm (100 GHz). In this case, it is necessary, according to the prior art hereabove mentioned, to adjust the length of the master laser with an accuracy of the order of a few Pico metres (a few GHz). According to the present invention, a wide-band source can be chosen, the width of which can be typically 0.4 nm (100 GHz), and the central wavelength of which can be adjusted around the targeted wavelength, but with an accuracy which will be only a fraction of the space between the wavelengths targeted for the slave laser.
In addition, in the advantageous case where the wide-band source emits a light beam comprising all the polarisation states, there will still be polarisation states which will not be perpendicular to the mode of the cavity, which makes possible to avoid the utilisation of a polarisation tester. In order to select a specific oscillation mode associated with said at least one wavelength, said injection means advantageously include an optical filter adapted to select at least one filtering wavelength among said plurality of emitting wavelengths. Said at least one filtering wavelength is, for example, obtained by specifying a central wavelength and space selectivity for the optical filter, these two lengths being, for example, tuneable.
In addition, in order to be able to modify said oscillation mode, said at least one filtering wavelength is variable. In addition, in order to protect said optical source against a disturbing lighting from the outside, said injection means may include, in addition, an optical isolator positioned at the output of said optical source. In addition, in order to be able to adjust the power of the beam derived from said optical source, said injection means may further include a variable attenuator. It should also be noted that in a way known per se, said laser includes an optical cavity and an amplifying medium.
In addition, in a particular embodiment, said means for injecting said light beam derived from said optical source into said at least one laser include an optical circulator. It should be noted that using such an optical circulator is advantageous for isolating said optical source from the light radiations emitted by said laser, and may make it possible not to use an optical isolator. In addition, according to another embodiment, said injection means include space switch means including at least one output and filtering means of said plurality of emitting wavelengths adapted to select at least one filtering wavelength among said plurality of emitting wavelengths, each said at least one output of said space switch means being connected to each of said lasers in order to selectively inject said light beam derived from said optical source into each of said lasers, said selection of each of said lasers, said selection of each of said lasers depending on said at least one filtering wavelength. In the latter embodiment, in the case of a plurality of said slave lasers, it is advantageous to be able to address the latter according to various wavelengths.
According to an embodiment, said filtering means are integrated in said switching means within a multiplexer-demultiplexer, preferably tuneable for adjusting the wavelengths associated to each output, as well as the space selectivity and/or a width of space selectivity. Other aims and advantages of the present invention will appear while reading the following detailed description.